1. Field of the Invention
The present invention relates to a surface acoustic wave element in which surface acoustic wave transducers respectively provided with interdigital electrodes having different electrode widths and periods in a direction of propagation of a surface acoustic wave are combined, and a filter using such a surface acoustic wave element.
2. Description of the Prior Art
Conventionally, in a general surface acoustic wave transducer having an interdigital electrode structure comprising positive and negative electrodes provided on a piezoelectric substrate (including a piezoelectric thin film substrate), the positive and negative electrodes are arranged at equal periods. FIG. 4(a) is a plan view showing a filter using conventional surface acoustic wave transducers, and FIG. 4(b) is a sectional view taken along the line X-Y of FIG. 4(a). An excitation-side surface acoustic wave transducer 41 transduces an electrical signal to a surface acoustic wave, and a reception-side surface acoustic wave transducer 42 detects the surface acoustic wave generated by the surface acoustic wave transducer 41 and propagating along an arrow A and transduces it to an electrical signal. In an interdigital electrode structure of each of the surface acoustic wave transducers 41 and 42, electrodes are arranged at equal periods. That is, p is constant and m/p is a constant (usually "0.5") where m is the electrode width and p is the period.
In the surface acoustic wave transducer having electrodes arranged at equal periods, a generated surface acoustic wave propagates to the right and left with substantially the same amplitude. Thus, this surface acoustic wave transducer has similar insertion loss characteristics in the two directions, i.e., the bi-directional characteristics.
Conventional techniques for obtaining a uni-directional surface acoustic wave transducer having a low insertion loss only in one direction by using a surface acoustic wave transducer in which electrodes are arranged at equal periods include a method of using a 120.degree.-phase shifter, a method of using a 90.degree.-phase shifter, and a method of using an internal reflection type uni-directional transducer for obtaining uni-directional characteristics by asymmetrically arranging reflecting electrodes between positive and negative electrodes at equal periods.
An example of a surface acoustic wave transducer is a dispersion type delay line (chirp filter). FIG. 5 is a plan view showing an arrangement of a conventional dispersion type delay line. An excitation-side surface acoustic wave transducer 51 transduces an electrical signal to a surface acoustic wave, and a reception-side surface acoustic wave transducer 52 detects the surface acoustic wave generated by the surface acoustic wave transducer 51 and propagating along an arrow B and transduces it to an electrical signal. In the interdigital electrode structure of the surface acoustic wave transducer 52, positive and negative electrodes 53 and 54 are alternately arranged such that their periods are gradually decreased in a direction B of propagation of the surface acoustic wave that is the positive and negative electrodes 53 and 54 have a graded periodicity. Characteristics in which the delay time changes in proportion to a frequency can be obtained by arranging the positive and negative electrodes in this manner.
A surface acoustic wave transducer in which electrodes are arranged at equal periods has bi-directional characteristics and cannot obtain a low uni-directional insertion loss. With the above technique of obtaining the uni-directional surface acoustic wave transducer, although a low uni-directional insertion loss can be obtained, since the surface acoustic wave transducer having electrodes arranged at equal periods is used, wide-band characteristics cannot be obtained.
According to the dispersion type delay line as shown in FIG. 5, the delay time of a signal having a high frequency is long, and the delay time of a signal having a low frequency is short, thus constituting a up-chirp delay line. A down-chirp delay line having opposite characteristics can also be obtained. In this case, however, since a conventional dispersion type transducer, i.e., a transducer having bi-directional characteristics is used, a transducer having a low insertion loss cannot be obtained.